Semiconductor devices such as a dynamic and nonvolatile random access memory ("DRAM") have decreased in size and increased in capacity dramatically over the last 20 years. As the capacity of memory cells has increased and the size has decreased, the design of the cells has become increasingly complex in order to preserve sufficient electrical capacitance to hold the electrical charge representing the stored data.
In the past silicon dioxide has been used as the dielectric in the capacitors of DRAM cells. However, silicone dioxide has a relatively low dielectric constant of about four. There has recently been experimentation with the use of other dielectric materials in order to increase the electrical capacitance in very small and complex cells. Some of this work has focused on the use of ferroelectric materials such as PZT as the dielectric in the capacitor. The relaxation properties of the PZT ferroelectric material produce an increase in the capacitance that is beyond what the dielectric constant would indicate. An example of such an approach is described in U.S. Pat. No. 5,109,357 by Eaton. Desirable materials for the electrodes associated with PZT capacitors include RuO.sub.2, Pt and ITO. RuO.sub.2 is especially desirable due to its high chemical stability, high electrical conductivity and its reduction of fatigue of PZT films. See, C. K. Kwok, D. P. Vijay and S. B. Desu, Proceedings of the 4th International Symposium on Integrated Ferroelectrics, Monterey, Calif. (1992).
Regardless of the material used as the dielectric in the capacitor of a DRAM, the cell surface must be patterned in some manner to produce the desired capacitor configuration. As mentioned, some of these capacitor configurations must be quite complex to achieve the necessary capacitance. PZT films have been patterned by laser-induced sputtering (see, M. Eyett, D. Bauerie, W. Wersing and H. Thomann, J. Appl. Phys. (62, 1511 (1987)), chemical wet etching (see, H. T. Chung and H. G. Kim, Ferroelectrics, 76 (1987)), ion milling and reactive ion etching (RIE). Preferably, the patterning technology can be performed at a rapid rate, produces a high resolution so that it can be used in complex capacitor configurations, and is highly uniform. RIE is particularly suitable because it produces a high etch rate at low etching power, high selectivity and good anisotropic profiles by appropriate selection of the reactive gas.
In ordinary RIE, material is selectively removed by an interaction with chemically reactive ion species created by a radio frequency ("RF") glow discharge maintained in the etching chamber. RIE normally involves covering the surface to be etched with a mask which leaves exposed the selected areas to be etched. The substrate is then placed into a chamber containing a chemically reactive gas such as CF.sub.4 mixed with O.sub.2. A plasma is produced by applying an RF potential across the gas to dissociate and form various species including positive and negative ions, reactive atoms such as fluorine, and radicals. This plasma reacts with the unmasked and exposed surface of the material to be etched to form volatile products which are removed to leave an etched profile.
One of the barriers to widespread use of PZT ferroelectrics as a dielectric in DRAM capacitors is the difficulty of etching such materials and their associate electrodes in an effective and efficient manner. The difficulty is compounded by the difficulty of identifying a suitable etch gas that can etch all three components of the PZT solid solution (PbO, ZrO.sub.2 and Ti.sub.1 O.sub.2) at an acceptable rate, and identifying a common etch gas for both the RuO.sub.2 electrode or other electrode and the PZT ferroelectric material which will allow for stacked capacitor etching. Plasma etching of PZT thin films in CF.sub.4 and HCl plasmas has been reported in M. R. Poor, A. M. Hurt, C. B. Fledermann and A. U. Wu, Mat. Res. Soc. Symp. Proc., 200 (1990). However, to obtain high etch rates, substrate heating was necessary in their process. RIE of RuO.sub.2 with a CF.sub.4 /O.sub.2 plasma has been reported in S. Saito and K. Kuramasu, Jpn. J. Appl. Phys. 31, 135 (1993). The use of CF.sub.4 +O.sub.2 has been reported in RIE of RuO.sub.2 (see, S. Saito and K. Kuramusa, Jpn. J. Appl. Phys. 31, 135 (1992)) and the use of CCl.sub.4 has been reported in RIE of PZT (sese, S. Saito, et al., Jpn. J. Appl, Phys. 31, L1260 (1992)). CCl.sub.2 F.sub.2 with O.sub.2 has been used to etch both RuO.sub.2 and PZT films, but CCl.sub.2 F.sub.2 is believed to be environmentally damaging. Therefore, there is a need for an environmentally safe etching gas to use in RIE of RuO.sub.2 and PZT films.